As disclosed in Japanese Patent No. 3405857, it is known that in order to avoid exhaust interference inside an integrating exhaust pipe section in an exhaust system in which two or more upstream side exhaust branch passages (pipes) for cylinders are connected to the entrance of the integrating exhaust pipe section, a partition plate is provided along the diameter direction of the integrating exhaust pipe section. For example, in a inline four cylinder internal combustion engine, an exhaust system includes upstream side exhaust branch passages that are joined so as to be in a so-called “4-2-1” form by dividing the inside of the integrating exhaust pipe section into two pathways by the partition plate, connecting upstream side exhaust branch passages for cylinders #1 and #4 to a first pathway, and upstream side exhaust branch passages for cylinders #2 and #3 to a second pathway.
Generally, in such a partition plate provided in the above described integrating exhaust pipe section, an upstream side end portion of the partition plate is fixed to and supported with an upstream side exhaust branch passage (such as pipes). A downstream side end portion of the partition plate, however, includes a non-connected or a free end within the integrating exhaust pipe section. Since the end is free, it tends to vibrate so that undesirable noise may be generated. As may be seen in Japanese Patent No. 3405857, in order to absorb the thermal expansion and heat contraction in a width direction of the partition plate, a curved portion is provided in a longitudinal direction of the partition plate, positioned generally at the center of the partition plate when measured with respect to the width direction. In such a structure, however, the rigidity of the partition plate becomes low and it further tends to vibrate, causing noise.